The vertebrate forebrain is one of the largest and most complex regions of the central nervous system (CNS) consisting of many of the organs necessary for higher cognitive functions. However, the molecule mechanisms that direct the regional specification, morphogenesis and differentiation of the vertebrate forebrain are only beginning to be understood. In order to elucidate the molecular events that initiate the development of the forebrain, I have chosen to study Nkx-2.1 and Nkx-2.2, two of the earliest known genes to be expressed in the forebrain. NKX-2.1 and NKX-2.2 are homeodomain- containing transcription factors that are expressed at the onset of neurulation in limited ventral domains of the developing forebrain in response to inductive signals from the secreted molecule sonic hedgehog, a known signaling protein. Detailed functional analyses of Nkx-2.1 and Nkx-2.2 should help to elucidate the intricate cascade of regulatory event that direct the morphogenesis of the vertebrate forebrain. Specifically, I propose: 1. To generate mice lacking either Nkx-2.1 or Nkx-2.2 in order to determine the roles that these genes play in the normal patterning of the embryonic forebrain. 2. To analyze the phenotype of the Nkx-2.1 and Nkx-2.2 mutant mice using histological staining methods, immunohistochemistry and RNA in situ hybridization in order to identify: a) the precise forebrain structures that require functional Nkx-2.1 or Nkx-2.2 for proper development; and b) when and where these genes are necessary for the appropriate differentiation of cell types. 3. To express Nkx-2.1 and Nkx-2.2 ectopically in specific regions of embryonic chick brain in order to determine the distinct tissues or populations of cells that develop in response to misexpression of these genes.